Sorry, you can't, as far as I know: https://mekoramaforum.com/media/button-function.14059/?page=2 (see Gepeto's comment)
In mekostudio builder, after the water is there, you can use the switch button to change the blocks back to grass: https://archive.org/download/autopilot_202006/switch.png (but, sorry, L-bots don't work with mekostudio).